1. Technical Field
This invention relates to CMOS relaxation oscillator circuits. More particularly, this invention relates to high speed CMOS relaxation oscillator circuits with reduced dependence on process and temperature variations.
2. Background
Oscillators are widely used throughout the microelectronics industry to provide a steady and stable periodic output waveform. For example, a typical oscillator could be used to generate the clock signals for digital applications. Another application where oscillator circuits are used is in the generation of an amplitude modulated carrier signal or a frequency modulated carrier signal for the transmission of data.
Oscillator circuits are generally broken down into two subclasses: tuned oscillators and relaxation oscillators.
Tuned oscillators have the advantage of high frequency stability, high frequency capability and higher spectral purity than relaxation oscillators. Tuned oscillators, however, require expensive and bulky components, such as inductors and crystals. Furthermore, these components are difficult to integrate into monolithic circuits. Additionally, tuned oscillators suffer from a narrow frequency band of operation.
Relaxation oscillators, however, have some unique advantages when used in integrated circuit design. For instance, relaxation oscillators do not require inductors or crystals. Thus, the relaxation oscillator can be easily incorporated into monolithic circuits. Additionally, the operative frequency band of a relaxation oscillator circuit is proportional to a charging current and inversely proportional to a voltage level across an energy storage device (e.g. capacitor) and the energy storage capacity of the energy storage device. The frequency of the relaxation oscillator circuit can easily be set using a single external component (e.g. a resistor) and can be varied linearly over a wide frequency band. A relaxation oscillator circuit, however, suffers from relatively poor frequency stability, particularly for high-speed operation. Furthermore, the relaxation oscillator circuit has a poor spectral purity characteristic.